The need for reducing or removing microorganisms contained in feed and/or exhaust air has been recently enhanced. Those generally called microbiologically clean rooms should be equipped with a device for sterilizing feed and/or exhaust air.
In particular, culture chambers for microorganisms, animal cells or plant cells and breeding rooms should be equipped with a cleaning device of a completely containment type.
For example, culture chambers for novel organisms obtained by a recombination of DNA or, pathogenic bacteria or viruses, and breeding rooms for animals having pathogenic bacteria should be equipped with a containment device on a P-3 or P-4 level. In these cases, it is necessary to remove all microorganisms, animal cells, and plant cells present in the exhaust air discharged from the culture chambers or breeding rooms.
On the other hand, food plants, pharmaceutical plants where microbiologically good manufacturing practice (generally referred to as GMP) is required, and operation rooms and infant rooms in hospitals should be equipped with devices for removing organisms in the feed air.
Sterilization of feed and/or exhaust air or microbiologically clean rooms and culture devices has been conventionally performed with a device comprising a filter layer having fine clearances. In exhaust systems wherein high security is required, filtration is carried out by using such a filter above-mentioned as a HEPA filter, being followed by incineration in an incinerator. However the removing ratio achieved by filtering is estimated to be 99.99.sup.+ % while that achieved by filtering followed by incineration is estimated to be 99.999.sup.+ %.
Different from other chemical contaminants, contaminating microorganism should be removed at a ratio of 100%. However this technical problem remains open.
Isao Endo has reported that the HEPA filter should be replaced by a more convenient and inexpensive filter and the frequency of its exchange should be examined in order to improve the treatment of exhaust air, thus proposing the necessity of establishing a sterilizing process (cf. Practice for culturing a large amount of DNA-recombined microorganisms", Kagaku Kogaku Technical Report, No. 3, the Chemical Industry Association, Nov. 10, 1983).
Under these circumstances, we have carried out studies on a process for effectively and readily sterilizing feed and/or exhaust air with a convenient device.
At first, we examined various processes for sterilizing feed and/or exhaust air, such as incineration, filtration, passage through a strong alkaline solution, passage through sterilizing shower, and ultraviolet sterilization. However each process so far tested has some disadvantages such that it requires large equipment which makes the process very expensive, that it is a tedious process or that it exhibits an insufficient sterilizing effect. In particular, the filter usually employed in the filter layer is expensive and tends to clog under highly humid conditions to thereby decrease the filter capacity. We have paid our attention to the incineration process which exhibits the highest sterilizing effect among these processes as described above and tried to overcome its disadvantages.
The incineration process is designed to perform in an incinerator or with an electric heater. Incineration in an incinerator makes it possible to treat feed and/or exhaust air at a high temperature.
However it has a disadvantage. That is, the residence time in the high-temperature zone is so short that the formed aerosol would pass therethrough in an unbroken state, thus leaving microorganisms intact in the feed and/or exhaust air though the number of the microorganisms thus left is very small.
On the other hand, in the case of an electric heater it was reported that complete sterilization can be achieved by using an extended heating part at a high temperature of approximately 500.degree. C. (cf. Hakko to Kogyo, vol. 42, No. 1, 9-15, 1984). However the result of our experiment revealed that in the case of a highly humid air, the aerosol might sometimes pass through even at such a high temperature in an unbroken state so that microorganisms are left intact as in the case of the incineration process with the use of an incinerator. It is preferable to employ a heating temperature as low as possible from both safety and economical viewpoints.